An inner liner is provided on an inside surface of a pneumatic tire as an air permeation inhibiting layer in order to hold air pressure of a tire constant. The inner liner is generally constituted of a rubber layer through which a gas is difficult to permeate, such as butyl rubber or halogenated butyl rubber. To thin the inner liner for the purpose of weight reduction of a tire, it is proposed that an air permeation resistant film having an islands-sea structure in which a continuous phase is formed of an air permeation resistant thermoplastic resin and a dispersed phase is formed of an elastomer component is used as an inner liner.
For example, PTL 1 mentioned below discloses that by melt-kneading a thermoplastic resin having an air permeation coefficient of 25×10−12 cc·cm/cm2·sec·cmHg or less and Young's modulus of more than 500 MPa and an elastomer having an air permeation coefficient of more than 25×10−12 cc·cm/cm2·sec·cmHg and Young's modulus of 500 MPa or less to perform dynamic crosslink, a continuous layer is formed of the thermoplastic resin and a dispersed phase is formed of the elastomer, thereby a film having an air permeation coefficient of 25×10−12 cc·cm/cm2·sec·cmHg or less and Young's modulus of from 1 to 500 MPa is obtained.
However, in the technology disclosed in PTL 1, because a thermoplastic resin having high rigidity constitutes a continuous phase (matrix), if it is tried to enhance flexibility of a film in order to improve moldability of a tire (that is, for example when it is tried to obtain a film having low Young's modulus such as 100 MPa or less), the proportion of an elastomer constituting the dispersed phase must be increased. However, where the proportion of the elastomer is increased, phase transition is easy to occur between the thermoplastic resin and the elastomer, and it becomes difficult to prepare a thin film having a thickness of, for example, 0.2 mm or less. As a result, excellent air permeation resistance due to the continuous phase formed of the thermoplastic resin becomes difficult to be developed.
PTL 2 mentioned below discloses a method for decreasing a particle size of an elastomer component which constitutes a dispersed phase (domain) using a material having high proportion of an elastomer, as one technique for enhancing flexibility. That is, PTL 2 discloses that by kneading and vulcanizing compounding ingredients under the conditions that a melt viscosity ratio of the compounding ingredients and volume fraction×melt viscosity ratio are constant in each kneading step using a two-stage kneading treatment, the dispersed phase can be formed of the elastomer component even in the range of the proportion of the elastomer exceeds 50 wt %, and its particle size can be decreased.
PTL 3 mentioned below discloses that for the same purpose as PTL 2, by a rubber composition comprising a rubber single substance as an elastomer component, and at least one compounding ingredient of a reinforcing agent, a plasticizer, an oil and a crosslinking agent, added thereto, and by using two components having the same composition and structure and having different viscosity, as thermoplastic resins, the relationship between those melt viscosities and volume fractions are specified into predetermined ranges.
Even in PTL 2 and PTL 3, a material constituting a continuous phase is a thermoplastic resin having high rigidity, similar to PTL 1, and PTL 1, PTL 2 and PTL 3 do not fundamentally solve the above-described problems.